Winetech

Project description

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PROJECT TITLE (IWBT 1/11)

Sources of acetic and other fatty acids and their role in sluggish or stuck fermentations.

PROJECT LEADER
Dr M du Toit

CO-WORKER(S)
Mr WJ du Toit
Dr MG Lambrechts
A Oelofse
L Downing

SUMMARY

To determine whether acetic acid bacteria, lactic acid bacteria, non-Saccharomyces yeasts or commercial wine yeasts are responsible for the above average acetic acid production in some red wine fermentations.

To advise the industry on the prevention of acetic acid production.

Incomplete or stuck fermentations are often experienced during red wine making. These wines often contain high levels of volatile acidity, which is detrimental to wine quality. The source(s) of these high acetic acid levels are unknown, as well as the role which acetic acid may play in the onset of a stuck fermentation. Numerous microorganisms can contribute to high levels of volatile acidity. Acetic acid bacteria produce acetic acid and ethyl acetate from sugar as well as ethanol in the presence of oxygen. Lactic acid bacteria can also produce acetic acid. Lactobacillus can be homofermentative (more than 85% lactic acid produced) or heterofermentative [lactic acid, carbon dioxide and ethanol (and/or acetic acid) in equimolar quantities produced]. Larger quantities of acetic acid would thus be produced in the presence of oxygen or any other oxidant instead of lactic acid and ethanol. Leuconostoc is heterofermentative and can also produce more acetate in the presence of oxygen. Certain Pediococcus species also form equimolar quantities of lactic and acetic acid from pentose sugars. Literature states that acetic and lactic acid bacteria produce acetic acid in the beginning of fermentation.

Certain wild yeasts, such as Brettanomyces, Hansenula anomala, Kloeckera apiculata and Candida krusei can produce high levels of acetic acid. These yeasts can occur at high numbers on the grapes, during fermentation and even in inoculated fermentations. Dry warm seasons might result in higher than normal amounts of these yeasts on grapes and could therefore be responsible for the high levels of acetic acid.

Commercial wine yeasts can also produce higher levels of acetic acid under certain conditions. These levels are influenced by the nitrogen source, sugar concentration and the pH. Higher levels of acetic acid are normally produced under growth-limiting conditions and also at sugar concentrations over 20%. The influence of the nitrogen differs according to its source. Certain compounds (ammonium, glutamate, asparagine) inhibit the production of acetic acid when an ample supply of nitrogen is present whilst others such as methionine and valine can enhance the production of acetic acid. When the available nitrogen is inadequate, higher concentrations of acetate are produced at higher sugar concentrations. At pH values lower than 3.2 and higher than 4, more acetic acid is formed.

No research has been done on the origin and sources of the high levels of acetic acid in South African wines. We believe that this research could provide answers to the short as well as long term problems being experienced annually with stuck fermentations and high levels of volatile acidity in red wine making.